An update to the firmware on my Airport Extreme base plus an update to Time Machine seems to equal Time Machine working (without a hack) to a USB drive on the router. Yay! I've been doing this for awhile on my MacBook Pro already, but it's sweet that it works now without the hack.
I'll have to find a big old drive on sale somewhere and put that baby on the router so I can have my media drive back in the MacBook Pro. Plus future Macs in the household can use it. ;)
I'm not saying TC will go away, but, like you said, it doesn't put the AEX owners in the dark. If I had $500 to blow, I'm not sure if I would buy a TC or go with an Airport and external drive. The reassurance of being able to replace the hard drive whenever I want sure is nice.